For DS1, when he was much younger and showing a strong interest in letters and number, it was his Vtech Alphabet Desk and his Fisher Price Calcubot. On the non-electronic side, it was, and still is, his megasketcher. He's on his second one.
For DS2, when he was 2, he wasn't playing at all appropriately with toys, so we chose toys that would reward him for actions. The fisher price piggy bank was great for that - in no time, he was putting the coins in the slot. We also tried the cookie jar, but the pieces were really hard for him to manipulate.
A toy my sister bought him for Christmas when he was tiny, a musical lion, ended up getting replaced when he puked all over it, since he loved it so much. He still plays with it and he's nearly 4! Now he's moved on to joining in with the songs - no words, but he knows all the tunes!
The sticklebrix DS1 got for his second birthday is still proving its worth - he's currently indulging his current interest in cars by building a road, with street lights, signposts and everything for his matchbox cars!